Backup kit for a patient-specific arthroplasty kit assembly

ABSTRACT

A method for preparing a backup kit for a plurality of patient-specific arthroplasty procedures scheduled at the same medical facility includes providing a database with data from completed arthroplasty procedures using patient-specific arthroplasty kits. The database includes comparisons between preoperatively planned implant size and intraoperatively implanted implant size. The method includes determining a statistically expected implant size deviation from a planned implant size for each implant included in a plurality of patient-specific arthroplasty kits prepared for a shipment to the medical facility using the database. A backup kit of backup implants is assembled for the shipment. The number and size of the backup implants is determined from the statistically expected implant size deviations.

INTRODUCTION

The present teachings provide a backup kit including backup arthroplastyimplants for a shipment of a plurality of patient-specific arthroplastykits for a plurality of corresponding arthroplasty procedures at thesame medical facility. The patient-specific arthroplasty kits generallyinclude patient-specific alignment guides, custom and non-customimplants and other instruments for use during an arthroplasty procedure.The patient-specific alignment guides (and patient-specific implants,when used) are designed and constructed preoperatively based onthree-dimensional digital images of the patient's joint that isscheduled to undergo arthroplasty. The digital images of the patient'sjoint can be reconstructed from medical scans of the patient usingcommercially available CAD (Computer Aided Design) and/or other imagingsoftware.

SUMMARY

The present teachings provide a method for preparing a backup kit for ashipment of patient-specific arthroplasty kits for correspondingarthroplasty procedures scheduled at the same medical facility. Themethod includes providing a database with data from completedarthroplasty procedures that were performed using patient-specificarthroplasty kits. The database includes comparisons betweenpreoperatively planned implant size and intraoperatively implanted(actual) implant size. A statistically expected implant size deviationfrom a planned implant size for each implant included in the shipment isdetermined using the database. A backup kit of backup implants isassembled for the shipment. The number and size of the backup implantsis determined from the statistically expected implant size deviations.

The present teachings also provide a backup kit for a plurality ofpatient-specific arthroplasty procedures scheduled at the same medicalfacility. The backup kit includes a plurality of backup implantsselected for a shipment of a plurality of patient-specific arthroplastykits for the medical facility. Each patient-specific arthroplasty kit isconfigured for corresponding arthroplasty procedures at the same medicalfacility. The number and size of the backup implants is determined fromstatistically expected implant size deviations from planned implantsizes in the corresponding arthroplasty kits using a compliancedatabase. The compliance database includes data from completedarthroplasty procedures performed using patient-specific arthroplastykits. The compliance database includes comparisons betweenpreoperatively planned implant sizes and intraoperatively implantedimplant sizes.

Further areas of applicability of the present teachings will becomeapparent from the description provided hereinafter. It should beunderstood that the description and specific examples are intended forpurposes of illustration only and are not intended to limit the scope ofthe present teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic view of an exemplary patient-specific arthroplastykit;

FIG. 2 is a schematic view of a shipment of a plurality ofpatient-specific arthroplasty kits similar to the patient-specificarthroplasty kit of FIG. 1 according to the present teachings;

FIG. 3 is a schematic view of a backup kit for the shipment of FIG. 2according to the present teachings;

FIG. 4 is a flowchart for a method according to the present teachings;and

FIG. 5 is an exemplary partial listing of a compliance databaseaccording to the present teachings.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DESCRIPTION OF VARIOUS ASPECTS

The following description is merely exemplary in nature and is in no wayintended to limit the present teachings, applications, or uses. Forexample, although some of the present teachings are illustrated for aknee implant, the present teachings can be used for any orthopedicimplant.

The present teachings provide a backup kit that includes backuparthroplasty implants for a shipment of a plurality of patient-specificarthroplasty kits for use at the same medical facility for a pluralityof corresponding arthroplasty procedures. The patient-specificarthroplasty kits generally include patient-specific guides, custom andnon-custom implants and other instruments for use during an arthroplastyprocedure such as, for example, cutting guides, drill guides, cuttingblocks, fixation pins, etc. The patient-specific guides and/orpatient-specific implants, if used, are designed and constructedpreoperatively based on three-dimensional digital images of thepatient's joint that is scheduled to undergo arthroplasty. The backupkit includes backup implants of various sizes to be included with eachparticular shipment. The number and sizes of the backup implants in thebackup kit are determined from a statistical analysis ofpatient-outcomes that are stored in a compliance database, as discussedbelow.

Generally, patient-specific devices including implants and/orpatient-specific alignment guides, resection guides or other instrumentscan be designed preoperatively using computer-assisted imaging methods.Three-dimensional digital images of the patient's joint anatomy can bereconstructed from MRI, CT, ultrasound, X-ray, or other medical scans ofthe patient's anatomy. Various CAD programs and/or other software can beutilized for three-dimensional digital image reconstruction, such as,for example, software commercially available from Materialise USA,Plymouth, Mich.

Various pre-operative planning methods and patient-specific devices aredisclosed in commonly assigned U.S. patent application Ser. No.11/756,057, filed May 31, 2007, U.S. patent application Ser. No.11/971,390, filed on Jan. 9, 2008, U.S. patent application Ser. No.12/025,414, filed on Feb. 4, 2008, U.S. patent application Ser. No.12/039,849, filed Feb. 29, 2008; U.S. patent application Ser. No.12/211,407, filed Sep. 16, 2008; U.S. patent application Ser. No.12/103,824, filed Apr. 16, 2008; U.S. patent application Ser. No.12/371,096, filed Feb. 13, 2009, U.S. patent application Ser. No.12/483,807, filed Jun. 12, 2009; U.S. patent application Ser. No.12/872,663, filed Aug. 31, 2010, U.S. patent application Ser. No.12/973,214, filed Dec. 20, 2010, and U.S. patent application Ser. No.12/978,069, filed Dec. 23, 2010. The disclosures of the above patentapplications are incorporated herein by reference.

In the preoperative planning stage for an arthroplasty procedure, apreoperative surgical plan is formulated for a specific patient withinteractive input from the patient's surgeon or other medicalprofessional. Imaging data of the relevant joint anatomy of a patientcan be obtained at a medical facility or doctor's office using any ofthe medical imaging methods described above. The imaging data caninclude, for example, various medical scans of a relevant joint portionor other relevant portion of the patient's anatomy, as needed for jointor other anatomy modeling and, optionally, for determination of animplant alignment axis or for other alignment purposes. The imaging datathus obtained and other associated information can be used to constructa three-dimensional computer (digital) image of the joint or otherportion of the anatomy of the patient, such as, the hip joint, kneejoint, etc. The three-dimensional digital image of the patient's anatomyis also used to formulate a preoperative surgical plan for the patientincluding, for example, location and orientation of resections, removalof osteophytes or other protrusions, mechanical alignment, deformitycorrection, ligament balancing, or other preoperative planned proceduresThe preoperative surgical plan can also include the design andconstruction of patient-specific alignment guides, resection guides orother patient-specific instruments, and, optionally, the design andconstruction of patient-specific implants. Further, the preoperativesurgical plan can include the determination and selection of particularsizes of non-custom implants, digital images of which can be viewed andcompared relative to the three-dimensional digital image of thepatient's anatomy on a digital display.

Generally, the patient-specific devices, whether implants orinstruments, are configured to match at least a portion of a jointanatomy of a specific patient and are generally designed and configuredusing computer modeling based on the patient's reconstructedthree-dimensional digital image of the patient's corresponding jointanatomy. Each patient-specific device includes a three-dimensionalpatient-specific surface that is configured to conformingly contact andmatch a corresponding surface of the patient (with or without cartilageor other soft tissue), using the reconstructed three-dimensional digitalimage of the patient's anatomy and the computer methods discussed above.In this respect, a patient-specific device can register and nestinglymate with the corresponding bone surface (with or without articularcartilage) of the specific patient in only one position.

The three-dimensional digital model of the patient's anatomy can beviewed on a computer display or other electronic screen and can also bereproduced as a hard copy on film or other medium and viewed by director indirect or backlight illumination. The digital model can be sizedfor viewing on any appropriate screen size (including handheld mobiledevices, such as smart phones, PDAs or tablets) and may be cropped,rotated, etc., as selected by the individual (e.g., the surgeon) viewingthe screen.

The surgeon's review of the surgical plan may include implant selection,and/or a request for one or more patient-specific instruments, such asalignment guides, resection guides or other instruments to be used withthe selected implant. The patient-specific devices can be manufacturedby rapid prototyping methods, such as stereolithography or other similarmethods or by CNC milling, or other automated or computer-controlledmachining or robotic methods. The patient-specific instruments, theselected implants and optionally other disposable instruments can besterilized, assembled in a patient-specific kit and forwarded to thesurgeon or the surgeon's medical facility for implantation. The selectedimplants can be non-custom implants of a size determined and approved bythe surgeon in the preoperative plan for the patient.

Many medical facilities (or surgeons) place orders requesting multiplepatient-specific kits for use by the same or different surgeons at thesame facility for procedures scheduled for the same day or a few daysapart. Referring to FIG. 2, an omnibus patient-specific kit assembly 200can include a plurality of patient-specific arthroplasty kits 100 (100A,100B, 100C, etc.) for shipment to a single medical facility. Eachpatient-specific arthroplasty kit 100A, 100B, 100C can be labeled withthe patient's name or identification number (ID), such as A, B, C, thecorresponding surgeon's name, X, Y, Z and the side of the joint for theprocedure, R or L for right or left, for example, or other information.Additionally, each patient-specific component in each patient-specificarthroplasty kit can also be labeled with the patient's name, the jointside, etc.

The present teachings provide a method to design and prepare a backupkit 300 (see FIG. 3) for an entire bulk order or shipment that includesseveral patient-specific arthroplasty kits, as discussed below.Accordingly, the present teachings reduce weight, expense andunnecessary backup implant components for each patient-specific kitwhile providing the surgeon(s) and or the facility with sufficientoversize or undersize implants for the totality of the procedures,rather than for each single procedure. The determination of the numberand size of additional or backup implants to be included in the backupkit 300 is made using statistical analysis of the size outcomes previousprocedures which are stored in a compliance database, as describedbelow. Duplicate size backup implants can also be included.

An exemplary patient-specific arthroplasty kit 100 for a knee procedureis shown in FIG. 1. It will be appreciated that similar kits can beprepared for other joints, such as the hip joint, shoulder joint orother joints. The patient-specific arthroplasty kit 100 can include, forexample, a patient-specific femoral alignment guide 110, apatient-specific tibial alignment guide 118, a femoral drill guide 112,a tibial drill guide 120, a femoral distal cutting block 114, a tibialcutting block 130, a four-in-one femoral cutting block 116, and a tibialtemplate 122. The patient-specific alignment guides 110, 118 aresingle-use guides and disposable. Detailed description of thepatient-specific alignment guides and associated methods can be found inthe patent applications referenced above (and incorporated by referenceherein). The various drill guides and cutting blocks can bere-sterilizable and reusable, although disposable drill guides, cuttingblocks and other tools can also be designed. Additionally, a set oftrochar pins 132 and a set of spring drill pins 134 can be included.Depending on the surgeon's preferences, additional tools and/or medicalproducts and supplies can be included, such as cutting blades, bonecement, biologics, etc.

The patient-specific arthroplasty kit 100 can include a set ofarthroplasty implants, such as, for example, a femoral implant 150, abearing 152 and a tibial implant 154, for an exemplary total kneearthroplasty procedure, or other implant components depending on thesurgical procedure and the surgeon's preference. The arthroplastyimplants can be either patient-specific or non-custom implants.Semi-custom arthroplasty implants, i.e., implants including somepatient-specific features and some standard, non-custom features canalso be used, as described, for example, in commonly assigned U.S.patent application Ser. No. 12/872,663, filed Aug. 31, 2010, referencedabove (and incorporated by reference herein).

Referring to FIGS. 1-3, the implants or implant components 150, 152, 154included in each patient-specific arthroplasty kit 100 can be non-customcomponents of a size selected during the preoperative plan for thepatient. Predominantly, the selected or planned implant size is found tobe a match and no change in implant size is required intraoperatively.For the few cases that the implant-size is found intraoperatively, forvarious reasons, not to be the best match, a best match implant can befound in the preoperatively prepared backup kit 300 with a degree of abuilt-in or preoperatively determined degree probability, using acompliance database 500, as discussed below. The backup kit 300 caninclude a predetermined number of backup implant components. Forillustration purposes only, three different sizes for each implantcomponent 150, 152, 154 are shown in the single backup kit 300 thataccompanies the patient-specific kit assembly 200. The selection ofnumber and sizes the implant components to be included in the backup kitis discussed below with reference to FIGS. 4 and 5.

An exemplary (but not necessary representative) portion of a compliancedatabase 500 is illustrated in FIG. 5. The compliance database 500 canbe generated from information collected for each arthroplasty procedurefor which a patient-specific arthroplasty kit was provided. Thecompliance database 500 can include the planned implant size, as well asthe actual implant size that was eventually used. Information collectedand provided by the database 500 can also include date of the procedure,patient Identification, surgeon name, the side (R right, L left) for theprocedure and other characteristics of the patient, such as, forexample, weight, ethnicity or race, gender, age or other characteristicsthat may be of significance in selecting implant size or that can befactors for a size deviation or mismatch.

The number of procedures scheduled for the same date can vary, the normbeing several procedures per day, although a single procedure or elevenor twelve procedures in a single day are not uncommon. In the exemplaryillustration of FIG. 5, the implant sizes presented are for a femoralcomponent 150 of the Vanguard® Knee Complete System, commerciallyavailable from Biomet Manufacturing Corp., Warsaw, Ind. The availablesizes correspond to mediolateral (ML) dimensions of the femur in mm. Theinterval between sizes for the Vanguard® Knee, for example, is 2.5 mmfor sizes ranging from size 55 to size 75. A size 80 mm is alsoavailable. Similarly, nine sizes are available for the tibial component154 and seven sizes for the bearing 152 for the Vanguarde Knee.Implanting the wrong size, i.e., implant mis-sizing for the femoralimplant 150 can result in excessive mediolateral overhang, especially inwomen, and can cause over-tightening of the knee capsule when a largersize is used. Providing backup sizes (one size larger or one sizesmaller than a planned size) for each and every implant in the shipmentto the same facility can be costly, wasteful and inefficient both inbulk and weight. The compliance database 500 can be created and used toreduce the number of backup implants for the shipment, while stillproviding a high probability that the intraoperatively determinedimplant size is the planned size or is included in the backup kit.

The partial data presented chronologically in FIG. 5 may be somewhatmisleading in that they show a higher proportion of mismatches betweenplanned and actual implant sizes than is the case when all the availabledata is considered for a longer period of time. For example, Applicant'sdata for a period ranging from Dec. 1, 2008 to Jun. 19, 2009 can besummarized as follows: 54 mismatches or implant size deviations for atotal of 397 arthroplasty procedures, or a 13.8% (54/398) mismatch (ordeviation) rate. From the 54 mismatches, in 44 mismatches the plannedsize was oversized (one size too big; rate 44/397=0.11) and in 11mismatches the planned size was undersized (one size too small; rate11/397=0.28). It is noted, however, that, according to the presentteachings, the compliance database 500 is updated frequently or for eachprocedure performed, and that the contents of the backup kit 300 canthen be determined by statistical analysis of the updated compliancedatabase 500. For example, if N patient-specific arthroplasty kits 100are required for a single day D in a medical facility M, and eachpatient-specific arthroplasty kit 100 includes one implant of the type150 (femoral knee component), then the database is analyzed to determinethe proportion of higher and lower sizes that may be needed for a set ofN implants of type 150 and having a particular size. The statisticalanalysis, in its simplest form, may be based on percentages, or mayinclude other considerations, such as reduction of mismatches over timeas the technology evolves, patient characteristics, surgeon preferences,various weighing factors and other parameters. The statistical analysiscan also include a specified contingency or probability coverage, i.e.,what percentage of all mismatch cases should be covered, such as, forexample from 98% or higher. Error analysis, standard deviation analysis,data reliability analysis and other statistical methods and algorithmsknown in the art can also be considered in the calculations. It is alsonoted that the compliance database 500 can depend on the characteristicsof the implants and instruments used and can, therefore, vary frommanufacturer to manufacturer. Accordingly, a relevant compliancedatabase 500 can be generated or otherwise available to be accessed fordetermining the backup kit for a particular manufacturer according tothe present teachings.

As an illustration, the surgical schedule for the date Dec. 8, 2008includes two R (right side) components of size 70 and three R componentsof size 67.5. For these five components, based on the 11% chance ofoversize determined from the compliance database 500, one backup implantof size R67.5 for planned implant size R70 and one backup implant ofsize R65 for planned size R67.5 may be provided in the backup kit 300.Duplicative backup implants (backup implant has same size as plannedimplant size) can also be included in the backup kit 300 to account forloss of sterilization, mishandling or other mishaps and accidents at themedical facility. The number of duplicative backup implants can bedetermined from statistical analysis of data included in the compliancedatabase 500, or from other historical records or databases, or fromsurgeon and/or medical facility preferences and requirements.

In some embodiments, the backup kit 300 may be prepared for all thearthroplasty procedures performed at the same medical facility M for adifferent period of time, a week, for example. For the week of Dec. 1,2008 to Dec. 5, 2008, for example, five L67.5 implants were planned. Onebackup implant of size L65 can be included in the backup kit based onthe 11% rate of oversize mismatch. If the undersize mismatch rate isonly 2.8%, as in data presented above, a separate backup kit 300 thatincludes bigger sizes for the lower chance of undersize planned implantsmay be maintained at facility M to be accessed as needed and replenishedperiodically. A backup kit with larger sizes for undersize plannedimplants can be prepared, for example, using estimates based onanticipated needs for longer periods of time, such as, per month,quarterly, etc. Further, the rate of mismatch can be calculated from thedatabase for each particular size. For example, if the rate of oversizemismatch is x % for a particular implant size N, then for every hundredimplants of size N (in mm) in one shipment (a patient-specific kitassembly 200) of patient-specific arthroplasty kits 100, an x number ofimplants of one size lower than N, i.e., N-2.5, can be included in thebackup kit 300. If only ten implants are included in the backup kit 300,then x/10 implants of the smaller size are included in the backup kit300. The statistically determined number of backup implants for eachplanned implant size in the shipment (or patient-specific kit assembly)200 of patient-specific arthroplasty kits 100 is not necessarily aninteger and can be rounded up or down to the nearest integer number.

It will be appreciated that other backup implants can be included in thebackup kit 300, above and beyond those that can be statisticallyrequired. For example, a surgeon may require a larger size backupimplant for any planned size below 60 mm. Such considerations can beused to modify the backup kit 300 after the backup implant componentshave been determined by statistical analysis. In some cases, thecontents determined by statistical analysis may already satisfy thesurgeon's or the medical facility's additional requirements.

Referring to FIG. 4, a flowchart of a method of assembling a backup kit300 according to the present teachings is illustrated. At block 400 acompliance database 500 is created or otherwise provided for access. Atblock 402, an omnibus patient-specific kit assembly or shipment 200 isprepared for a facility M. As discussed above, the patient-specific kitassembly 200 includes a plurality of patient-specific arthroplasty kits100 that are scheduled to be performed on the same day or within aspecified number of days, such as, for example, on the same forthcomingweek. The sizes of the various implants of the patient-specific kitassembly 200 and corresponding patient-characteristic can be identifiedat block 404. A statistical analysis can be performed at block 406 todetermine the likelihood of deviation or mismatch between preoperativelyplanned implant sizes and actual implant sizes as predicted by thecompliance database 500. Based on the results of the statisticalanalysis, expected deviations in implant size, i.e., the number ofplanned implants expected to be oversized or undersized can bedetermined at block 408 and a backup kit of implants can then beassembled at block 410. The compliance database 500 can be updated at412 with data from the new arthroplasty procedures that have beenperformed.

As discussed above, a compliance database 500 of preoperatively plannedand actual (intraoperatively selected) implants can be created from datacollected from patient-specific preoperative plans and actualarthroplasty usage. The database provides details of mismatches ordeviations between a planned implant size and an actually used implantsize. The information in the database can be statistically analyzed tohelp determine the number and sizes of extra of backup implants to beincluded in a backup kit 300 for a plurality of arthroplasty proceduresthat are planned preoperatively and include patient-specificarthroplasty kits 100.

Example embodiments are provided so that this disclosure is thorough,and fully conveys the scope to those who are skilled in the art.Numerous specific details are set forth, such as examples of specificcomponents, devices, and methods, to provide a thorough understanding ofembodiments of the present disclosure.

It will be apparent to those skilled in the art that specific detailsneed not be employed, that example embodiments may be embodied in manydifferent forms and that neither should be construed to limit the scopeof the disclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail. Accordingly, individual elements or features of aparticular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

1-20. (canceled)
 21. A backup system for a plurality of patient-specific arthroplasty procedures scheduled at the same medical facility for a plurality of different patients comprising: a plurality of implants selected and assembled for a shipment of a plurality of patient-specific arthroplasty systems for the medical facility, the plurality of patient-specific arthroplasty systems configured for corresponding arthroplasty procedures at the same medical facility for a planned period of time, at least one of the plurality of patient-specific arthroplasty systems having one or more of the plurality of implants with a size that differs from a planned size of another of the plurality of backup implants that are part of the plurality of patient-specific arthroplasty systems, a number and sizes of at least one of the plurality of implants determined from a statistically expected implant size deviation from a planned implant size in the plurality of patient-specific arthroplasty systems using a database having data from completed arthroplasty procedures with other patient-specific arthroplasty systems, the database including comparisons between preoperatively planned implant sizes and intraoperatively implanted implant sizes.
 22. They system of claim 21, wherein the one or more of the plurality of implants comprise one or more backup implants designed to be used instead of the another of the plurality of backup implants should the another of the plurality of backup implants not fit one of the plurality of different patients properly.
 23. The system of claim 21, wherein the size of the one or more of the plurality of backup implants differs from the planned size of another of the plurality of backup implants by an interval of 2.5 mm.
 24. The system of claim 21, wherein the plurality of patient-specific arthroplasty systems each include a patient-specific alignment guide.
 25. The system of claim 24, wherein the patient-specific alignment guide comprises a patient-specific knee alignment guide.
 26. The system of claim 25, wherein the plurality of patient-specific arthroplasty systems each include a non-custom femoral implant, a non-custom tibial bearing component and a non-custom tibial implant.
 27. The system of claim 21, wherein the planned period of time comprises one of a single day or a single week.
 28. A backup system for a plurality of patient-specific arthroplasty procedures scheduled at the same medical facility for a plurality of different patients comprising: a database of data from completed arthroplasty procedures using prior used patient-specific arthroplasty systems, information collected and provided by the database including comparisons between a plurality of preoperatively planned implant sizes and a plurality of intraoperatively implanted implant sizes, the database being updated after the patient-specific arthroplasty procedures are performed and depending upon the characteristics of implants and instruments used in the plurality of patient-specific arthroplasty procedures; an omnibus backup system of backup implants assembled for the shipment by accessing and referencing the database, wherein the omnibus backup system includes a plurality of patient-specific arthroplasty systems; and a computer configured to determine a statistically expected implant size deviation from a planned implant size for each implant included in the plurality of patient-specific arthroplasty systems for the plurality of different patients that form the omnibus backup system during a planned time period prepared for a shipment to the medical facility using the database, the computer determining of the statistically expected implant size deviation being performed utilizing algorithms for statistical analysis; wherein a number and size of the backup implants are determined from the statistically expected implant size deviations and a number of implants corresponding to the planned implant size based on the totality of procedures for the plurality of different patients during the planned time period.
 29. The system of claim 28, wherein the database is updated with intraoperatively implanted implant size for each patient-specific arthroplasty system after a corresponding arthroplasty procedure is completed.
 30. The system of claim 28, wherein each of the plurality of patient-specific arthroplasty systems includes a patient-specific alignment guide.
 31. The system of claim 28, wherein each of the plurality of patient-specific arthroplasty systems includes a non-custom implant.
 32. The system of claim 28, wherein the size of at least one backup implant differs by a size interval of 2.5 mm from the planned implant size.
 33. The system of claim 28, wherein the computer is configured to determine a quotient of the rate of mismatch divided by the number of implants corresponding to the preoperatively planned implant sizes.
 34. The system of claim 28, wherein the computer is configured to determine a proportion of the backup implants having larger and smaller sizes than the preoperatively planned implant sizes based on the rate of mismatch and the number of implants corresponding to the plurality of the preoperatively planned implant sizes.
 35. The system of claim 34, wherein the rate of mismatch includes a rate of oversized mismatch and a rate of undersized mismatch.
 36. The system of claim 34, wherein the statistically expected size deviation includes an oversized deviation and an undersized deviation.
 37. The system of claim 28, wherein the number and size of the backup implants are determined based on a percentage of mismatch cases to be accounted for. 